Reactive ion etching is a process whereby a low pressure gas is subject to a radio frequency electric field in a reaction chamber to form a plasma. A plasma is a gas which contains positive, negative and neutral atoms, and/or molecules including radicals and a “gas” of emitted photons. The ions and radicals in the plasma that form the etchants are accelerated by an electric field against the material to be etched. The ions/radicals interact with the surface of the atoms or molecules within the material to be etched, forming a volatile by-product which is subsequently removed from the reaction chamber.
If a chemically inert gas, such as argon, is ionized and accelerated to impinge on a substrate surface, material can be removed from the surface of the substrate by momentum transfer, a process similar to sand blasting. This process is used in three distinct modes: sputter etching, ion-beam milling and focused ion beam etching. Sputter etching and broad-ion beam milling use high-energy, inert gas ions (typically Ar+) to dislodge material from the substrate surface, a highly anisotropic etch process. Anisotropic etching occurs when the etch rate is considerably greater in one direction then in another (also known as unidirectional etching). Isotropic etch refers to etching in all directions at a relatively even rate.
The inherent poor selectivity and slow etch rate of these purely physical processes, however, severely limit their use in the cleaning of sub-micron patterns. It can be difficult to focus plasma to the bottom of high aspect ratio (depth:width) features. The anisotropic nature of dry etching makes it difficult to clean two or more surfaces. For example, it is often desirable to clean the bottom and side walls of a high aspect ratio feature on a substrate. Plasma reactors are difficult to modify to perform both the etching process and metal deposition. Finally, the sputtered material is typically non-volatile and tends to re-deposit onto the substrate and elsewhere in the system.
The ion bombardment in reactive ion etching can also result in a charge build-up on insulated surfaces, resulting in damage to the underlying film and semiconductor surface exposed to ion bombardment. For example, if the beam strikes a conducting grounded surface, sufficient secondary electrons are generated to balance the space charge of the beam and external neutralization is not necessary. If ions impinge on an insulated surface, however, positive charge can build-up on the surface, damaging the underlying insulator and semiconductor surface. When accumulated surface charge causes excessive current to pass through an insulator, the damage to the dielectric can be permanent.
Wet process cleaning to remove native oxides is limited in that the surface tension of the liquid inhibits penetration down into surface features with small lateral dimensions or high aspect ratios. Wet process cleaning is difficult to control. Finally, it is difficult to integrate wet process cleaning in the same equipment with metal deposition.